1. Field of the Invention
The present invention relates to an apparatus method for electrically measuring properties of a semi-conductor wafer without contaminating the semi-conductor wafer.
2. Description of the Related Art
Measuring the properties of semi-conductor wafers is a procedure carried out generally for the purpose of inspecting the quality of the semi-conductor wafers. Electrical measurements, such as C-V measurement and C-t measurement, are typically applied to determine the properties of semi-conductor wafers.
A C-V measurement apparatus disclosed in U.S. Pat. No. 5,233,291 is one type of conventional apparatus used for such electrical measurement of semi-conductor wafers. This apparatus measures properties of semi-conductor wafers according to the C-V measurement process. Structure and operations of this apparatus are described below with reference to FIG. 1 which is a schematic showing a conventional C-V measurement apparatus used for electrical measurement of semi-conductor wafers.
As illustrated in FIG. 1, the apparatus includes a fixed bench 1, a piezoelectric actuator 2 disposed below the fixed bench 1, and a base 3 further disposed below the piezoelectric actuator 2. A prism 4 arranged on a bottom face of the base 3 includes two prism elements of truncated cone shape, whose bases are opposed to and in contact with each other. A laser source 5 like GaAlAs laser is fixed on one inclined face of the base 3 whereas a photosensor 6 like a photo diode is fixed on the other inclined face of the base 3.
A metal stage 7, which is composed of a metal, preferably stainless steel, and used for supporting a semi-conductor wafer 20, is mounted on a fixed table 9 disposed below the prism 4. A bottom face 4a of the prism 4 is arranged to be parallel to an XY plane, which is parallel to the surface of the metal stage 7. The surface of the semi-conductor wafer 20 mounted on the metal stage 7 is thus substantially parallel to the bottom face 4a of the prism 4.
Through holes 7a and 9a are formed in the metal stage 7 and the table 9, respectively, and connect with an oil rotary vacuum pump 8 via a pipe 10. This means that the through holes 7a and 9a and the pipe 10 form a vacuum line. Operation of the oil rotary vacuum pump 8 allows the semi-conductor wafer 20 mounted on the metal stage 7 to be sucked onto the metal stage 7.
A ring electrode 15 is fixed to the bottom face 4a of the prism 4. An underside of the ring electrode 15 is apart by a gap G from the surface of the semi-conductor wafer 20.
As described in detail in the aforesaid U.S. Pat. No. 5,233,291, the C-V measurement apparatus measures the gap G by utilizing the tunnel effect of a laser beam totally -reflected at the bottom face 4a of the prism 4 in geometric optical total reflections. In the tunnel effect, when there is a sufficiently short distance between the bottom face 4a of the prism 4 and the surface of the semi-conductor wafer 20, part of a laser beam leaks to the air gap between the prism 4 and the semi-conductor wafer 20.
The piezoelectric actuator 2 is connected to a Z-directional position control unit 11. The piezoelectric actuator 2 receives a voltage applied from the Z-directional position control unit 11 and shifts the base 3 in the Z or vertical direction.
The photosensor 6 is connected with an actinometer 12 while the electrode 15 and the metal stage 7 are connected to an impedance meter 13 which measures a combination of capacitance between the electrode 15 and the metal stage 7.
The Z-directional position control unit 11, the actinometer 12, and the impedance meter 13 are connected to a host controller 14. The host controller 14 is, for example, a personal computer which controls the whole measurement apparatus and executes processing of data obtained.
The C-V measurement apparatus thus constructed sets the gap G between the electrode 15 and the semi-conductor wafer 20 mounted on the metal stage 7 equal to a predetermined value by means of the Z-directional position control unit 11 and the actinometer 12, and executes the C-V measurement by means of the impedance meter 13, while the electrode 15 and the metal stage 7 function as measurement electrodes.
The C-V measurement apparatus can perform the C-V measurement without requiring formation of any additional electrodes on an insulating film of the semi-conductor wafer 20 as a result becomes.
In the conventional C-V measurement apparatus, the semi-conductor wafer 20 is mounted on the metal stage 7, so that the rear face of the semi-conductor wafer 20 is in direct contact with the surface of the metal stage 7 and thereby contaminated with the metal included in the metal stage 7, such as Ni (nickel), Cr (chromium), or Fe (iron).
If the metal-contaminated semi-conductor wafer is placed with other non-contaminated semi-conductor wafers in a sealed vessel in a heating process, metal atoms attached to the rear face of the metal-contaminated semi-conductor wafer diffuse into the atmosphere in the sealed vessel. These diffused metal atoms then adhere to an opposite face Of the contaminated semi-conductor wafer and surfaces of the other non-contaminated semi-conductor wafers, resulting in undesirable propagation of metal contamination.
Once the rear face of a semi-conductor wafer is contaminated with metal, the semi-conductor wafer cannot be processed further to manufacture semi-conductor products. Because of this, dummy semi-conductor wafers rather than real products of semi-conductor wafers are utilized for electrical measurement with the conventional apparatus.
When electrical measurement of various properties of semi-conductor wafers is required at such step of the manufacturing process, a large number of dummy semi-conductor wafers should be prepared, which significantly increases cost.